Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder.
Further, engines have been proposed using more than one type of fuel injection. For example, the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. are one example. Specifically, the Heywood et al. papers describe directly injecting ethanol to improve charge cooling effects, while relying on port injected gasoline for providing the majority of combusted fuel over a drive cycle. The ethanol provides increased octane and increased charge cooling due to its higher heat of vaporization compared with gasoline, thereby reducing knock limits on boosting and/or compression ratio. Further, water may be mixed with ethanol and/or used as an alternative to ethanol. The above approaches purport to improve engine fuel economy and increase utilization of renewable fuels.
However, engines operated on alcohols, such as ethanol fuel, may be more prone to preignition or surface ignition than engines operated on gasoline. The source of preignition is often the spark plug, and thus for vehicles using a relatively consistent amount of ethanol or another alcohol fuel (e.g., pure ethanol, or a controlled blend, such as E85) one potential solution is to use a spark plug design having a lower heat range to operate at reduced temperatures.
However, the inventors herein have recognized a disadvantage with such an approach when the engine combustion chamber may receive varying ratio of fuel types. For example, under conditions where knock limits on spark advance are not restrictive, the cylinders may operate with a lower alcohol amount, whereas under conditions where knock limits on spark advance may cause fuel economy losses, the cylinders may operate with a higher alcohol amount to suppress knock and reduce limits on spark advance. In such cases, a higher temperature spark plug design may cause pre-ignition during the conditions of increased alcohol. Alternatively, a lower temperature spark plug design may cause spark plug fouling during the conditions of decreased alcohol.
In other words, the selection of spark plug heat range may be a trade-off between the risk of preignition at high loads and the risk of spark plug carbon fouling at light loads. The proposed combination of ethanol at high loads and gasoline at low loads, for example, makes this trade-off much more difficult, because ethanol is more prone to preignition than gasoline, and gasoline is more prone to spark plug carbon fouling than ethanol.
This paradox may be addressed by a system for an engine of a vehicle, comprising at least one combustion chamber located in the engine; a delivery system configured to deliver a fuel and a fluid to the combustion chamber; an ignition system including a spark plug configured to ignite the fuel within the combustion chamber; a spark plug heating system configured to supply heat to the spark plug; and a control system configured to vary an amount of heat supplied to the spark plug by the spark plug heating system responsive to a condition of the ignition system.
Thus, in one example, the engine may be configured to use a lower heat range spark plug to reduce pre-ignition when using variable amounts of gasoline and an alcohol (such as ethanol), for example. Further, issues with spark plug fouling of the lower heat rang plug may be addressed by varying an amount of heat supplied to a spark plug (such as during fouling conditions) to thereby decrease the likelihood of fouling.